A microelectromechanical system (MEMS) is a microscopic device with moving parts that is fabricated in the same general manner as integrated circuits. For example, a MEMS microphone is a transducer that converts sound into an electrical signal. Among different designs of microphone, a capacitive microphone or a condenser microphone is conventionally constructed employing the so-called “parallel-plate” capacitive design. Unlike other microphone types that require the sound wave to do more work, only a very small mass in capacitive microphones needs be moved by the incident sound wave. Capacitive microphones generally produce a high-quality audio signal and are now the popular choice in consumer electronics, laboratory and recording studio applications, ranging from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones.
FIG. 1A is a schematic diagram of parallel capacitive microphone in the prior art. Two thin layers 101 and 102 are placed closely in almost parallel. One of them is fixed backplate 101, and the other one is movable/deflectable membrane/diaphragm 102, which can be moved or driven by sound pressure. Diaphragm 102 acts as one plate of a capacitor, and the vibrations thereof produce changes in the distance between two layers 101 and 102, and changes in the mutual capacitance therebetween.
“Squeeze film” and “squeezed film” refer to a type of hydraulic or pneumatic damper for damping vibratory motion of a moving component with respect to a fixed component. Squeezed film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component (e.g., between approximately 2 and 50 micrometers). The squeezed film effect results from compressing and expanding the fluid (e.g., a gas or liquid) trapped in the space between the moving plate and the solid surface. The fluid has a high resistance, and damps the motion of the moving component as the fluid flows through the space between the moving plate and the solid surface.
In capacitive microphones as shown in FIG. 1A, squeeze film damping occurs when two layers 101 and 102 are in close proximity to each other with air disposed between them. The layers 101 and 102 are positioned so close together (e.g. within 5 μm) that air can be “squeezed” and “stretched” to slow movement of membrane/diaphragm 101. As the gap between layers 101 and 102 shrinks, air must flow out of that region. The flow viscosity of air, therefore, gives rise to a force that resists the motion of moving membrane/diaphragm 101. Squeeze film damping is significant when membrane/diaphragm 101 has a large surface area to gap length ratio. Such squeeze film damping between the two layers 101 and 102 becomes a mechanical noise source, which is the dominating factor among all noise sources in the entire microphone structure.
Perforation of backplate has been employed to control the squeeze film damping to a desired range. For example, US Patent Application 2014/0299948 by Wang et al. discloses a silicon based MEMS microphone as shown in FIG. 1B. Microphone 10 may receive an acoustic signal and transform the received acoustic signal into an electrical signal for the subsequent processing and output. Microphone 10 includes a silicon substrate 100 and an acoustic sensing part 11 supported on the silicon substrate 100 with an isolating oxide layer 120 sandwiched in between. The acoustic sensing part 11 of the microphone 10 may include at least: a conductive and compliant diaphragm 200, a perforated backplate 400, and an air gap 150. The diaphragm 200 is formed with a part of a silicon device layer such as the top-silicon film on a silicon-on-insulator (SOI) wafer or with polycrystalline silicon (Poly-Si) membrane through a deposition process. The perforated backplate 400 is located above the diaphragm 200, and formed with CMOS passivation layers with a metal layer 400b imbedded therein which serves as an electrode plate of the backplate 400. The air gap 150 is formed between the diaphragm 200 and the backplate 400. The conductive and compliant diaphragm 200 serves as a vibration membrane which vibrates in response to an external acoustic wave reaching the diaphragm 200 from the outside, as well as an electrode. The backplate 400 provides another electrode of the acoustic sensing part 11, and has a plurality of through holes 430 formed thereon, which are used for air ventilation so as to reduce air damping that the diaphragm 200 will encounter when starts vibrating. Therefore, the diaphragm 200 is used as an electrode plate to form a variable condenser 1000 with the electrode plate of the backplate 400. The acoustic sensing part 11 of the microphone 10 may further include an interconnection column 600 provided between the center of the diaphragm 200 and the center of the backplate 400 for mechanically suspending and electrically wiring out the diaphragm 200 using CMOS metal interconnection method, and the periphery of the diaphragm 200 is free to vibrate.
This structure typically contains a series of tiny holes or tiny slots, for example, on the edge of diaphragm, in order to control the resistance of air flow in a desired level. This air flow is between the two sides of diaphragm and is also called air leakage. When the air leakage rate is too low, the air pressure on the two sides of the diaphragm might be unbalanced. Consequently, a sudden air pressure change or a sudden acceleration of the microphone may cause a sudden motion of moving membrane/diaphragm 101, which may damage the delicate membrane/diaphragm 101. When the air leakage rate is too high, the microphone may have a descending sensitivity response on low frequency audio. Advantageously, the present invention provides a solution to such a problem with a new design of air flow restrictor, in which the air leakage is controlled to a desired range, i.e. not too high and not too low.